1. Technical Field
The present disclosure relates to a microelectromechanical structure, in particular a biaxial or triaxial gyroscope, provided with a rotary driving movement and improved electrical characteristics, in particular in terms of sensitivity in the detection of angular velocities.
2. Description of the Related Art
As is known, micromachining techniques enable manufacturing of microelectromechanical structures or systems (MEMS) within layers of semiconductor material, which have been deposited (for example, a polycrystalline-silicon layer) or grown (for example, an epitaxial layer) on sacrificial layers, which are removed via chemical etching.
Inertial sensors, accelerometers, and gyroscopes obtained with this technology are having an increasing success, for example, in the automotive field, in inertial navigation, or in the field of portable devices.
In particular, known to the art are integrated gyroscopes made of semiconductor material obtained with MEMS technology. These gyroscopes operate on the basis of the theorem of relative accelerations, exploiting the Coriolis acceleration. When an angular velocity is applied to a mobile mass that moves at a linear velocity, the mobile mass “feels” an apparent force, referred to as “Coriolis force”, which causes a displacement thereof in a direction perpendicular to the direction of the linear velocity and to the axis about which the angular velocity is applied. The mobile mass is supported via springs that enable a displacement thereof in the direction of the apparent force. On the basis of Hooke's law, the displacement is proportional to the apparent force, in such a way that from the displacement of the mobile mass it is possible to detect the Coriolis force and the value of the angular velocity that has generated it. The displacement of the mobile mass can, for example, be detected in a capacitive way by determining, in resonance conditions, the variations of capacitance caused by the movement of mobile electrodes, fixed with respect to the mobile mass and combfingered with fixed electrodes.
The published US patent applications US 2007/0214883, US 2009/0064780, and US 2009/0100930, filed by the applicant of the present application, disclose an integrated microelectromechanical sensor with rotary driving movement and sensitive to angular velocities of pitch, roll, and yaw about respective axes of detection.
This microelectromechanical sensor comprises a single driving mass, anchored to a substrate in a single central point, and actuated with rotary motion about an axis passing through the central point and orthogonal to the plane of the driving mass. The movement of rotation of the driving mass makes it possible to obtain in the plane of the mass two components of driving velocity orthogonal with respect to one another. Through openings are provided within the driving mass, and corresponding sensing masses are arranged in the through openings; the sensing masses are enclosed in the overall dimensions of the driving mass, are suspended with respect to the substrate, and are connected to the driving mass via flexible elements. Each sensing mass is fixed with respect to the driving mass during its rotary motion, and has a further degree of freedom of movement as a function of an external stress, in particular a Coriolis force, acting on the sensor. The flexible elements, thanks to their particular construction, enable the sensing masses to perform a rotary movement of detection about an axis belonging to the plane of the sensor, or alternatively a linear movement of detection along an axis belonging to the plane of the sensor, respectively, in response to a Coriolis acceleration acting in a direction perpendicular to the plane and to a Coriolis acceleration acting in a direction belonging to the plane. The movement of detection is in any case substantially decoupled from the movement of actuation of the driving mass. This microelectromechanical structure, in addition to being compact (in so far as it envisages a single driving mass enclosing in its overall dimensions a number of sensing masses), makes it possible to obtain with minor structural modifications, a uniaxial, biaxial, or triaxial gyroscope (and/or possibly an accelerometer, according to the electrical connections implemented), at the same time ensuring an excellent decoupling of the driving dynamics from the detection dynamics.
FIG. 1 shows an exemplary embodiment of a triaxial microelectromechanical gyroscope, designated by 1, according to the teachings contained in the aforesaid patent applications.
The gyroscope 1 is made in a die 2, comprising a substrate 2a made of semiconductor material (for example, silicon), and a frame 2b defining inside it an open region 2c; the open region 2c overlies the substrate 2a, and is designed to house detection structures of the gyroscope 1 (as described in detail hereinafter). The open region 2c has a generally square or rectangular configuration in a horizontal plane (in what follows, plane of the sensor xy), defined by a first horizontal axis x and a second horizontal axis y, fixed with respect to the die 2. The frame 2b has sides that are substantially parallel to the horizontal axes x, y. Contact pads 2d (so-called “die pads”) are arranged along a side of the frame 2b, aligned, for example, along the first horizontal axis x. In a way not illustrated, the die pads 2d enable electrical contact from outside of the detection structures of the gyroscope 1. The die pads 2d have an axis of symmetry, in this case coinciding with the second horizontal axis y (orthogonal to their direction of alignment), and are arranged in equal number and in a specular way on opposite sides of the second horizontal axis y.
In particular, the first and second horizontal axes x, y correspond to a first axis of detection and to a second axis of detection of the gyroscope 1 (more precisely, to a pitch axis and to a roll axis), about which corresponding pitch and roll angular velocities {right arrow over (Ω)}x and {right arrow over (Ω)}y are detected.
In detail, the gyroscope 1 comprises a driving structure, housed within the open region 2c and including a driving mass 3 and a driving assembly 4.
The driving mass 3 has a generally circular geometry with radial symmetry, with a substantially planar configuration having a main extension in the plane of the sensor xy, and a negligible dimension, with respect to the main extension, in a direction parallel to a vertical axis z, forming with the first and second horizontal axes x, y a set of three orthogonal axes, fixed with respect to the die 2. For example, the driving mass 3 has in the plane of the sensor xy basically the shape of an annulus, and defines at the center an empty space 6, the center O of which coincides with the center of gravity and the center of symmetry of the entire structure.
The driving mass 3 is anchored to the substrate 2a by means of a first anchorage 7a set in an area corresponding to the center O, to which it is connected through first elastic anchorage elements 8a. In the example, the first elastic anchorage elements 8a depart, forming a cross, from the center O, parallel to the first and second horizontal axes x, y. The driving mass 3 is anchored to the substrate 2a by means of further anchorages 7b, set on the outside of the same driving mass 3, to which it is connected by means of further elastic anchorage elements 8b. For example, the further elastic anchorage elements 8b are of the folded type, are four in number, and are set aligned in pairs along the first and second horizontal axes x, y; accordingly, the further anchorages 7b are arranged, in pairs, on opposite sides of the driving mass 3 with respect to the empty space 6, at the ends of a cross centered in the center O. The first and further elastic anchorage elements 8a, 8b enable a rotary movement of the driving mass 3 about an axis of actuation passing through the center O, parallel to the vertical axis z and perpendicular to the plane of the sensor xy.
The driving mass 3 has: a first pair of first through openings 9a, 9b, aligned in a diametric direction along the first horizontal axis x (pitch axis), and set on opposite sides with respect to the empty space 6; and a second pair of first through openings 9c, 9d, aligned in a diametric direction along the second horizontal axis y (roll axis), and set on opposite sides with respect to the empty space 6. In particular, each of the first through openings 9a-9d has in the plane of the sensor xy the shape of a radial sector of an annulus, having arc-shaped internal and external sides and radially-extending lateral sides. In addition, the through openings 9a, 9b of the first pair are symmetrical with respect to the second horizontal axis y, and the through openings 9c, 9d of the second pair are symmetrical with respect to the first horizontal axis x. Moreover, the driving mass 3 has a pair of second through openings 26a, 26b, having in plan view a substantially rectangular shape, aligned in a radial direction (in the example of FIG. 1 in a direction inclined by 45° with respect to the first horizontal axis x or the second horizontal axis y), and having a main extension in the same radial direction.
The driving assembly 4 comprises a plurality of driven arms 10, extending externally from the driving mass 3 in a radial direction and in such a way that they are set at equal angular distances apart, and a plurality of first and second driving arms 12a, 12b, which extend parallel to, and on opposite sides of, respective driven arms 10. Each driven arm 10 carries a plurality of first electrodes 13, extending perpendicular to, and on either side of, the same driven arm. In addition, each of the first and second driving arms 12a, 12b carries respective second electrodes 14a, 14b, which extend towards the respective driven arm 10 and are combfingered with the corresponding first electrodes 13. The first driving arms 12a are arranged all on one and the same side of the respective driven arms 10, and are biased all at one and the same first voltage. Likewise, the second driving arms 12b are arranged all on the opposite side of the respective driven arms 10, and are biased all at one and the same second voltage. A driving circuit (not illustrated) is connected to the second electrodes 14a, 14b for applying the first and second voltages and determining, by means of the mutual and alternating attraction of the electrodes, an oscillatory rotary movement of the driving mass 3 about the axis of actuation, at a given frequency of oscillation.
The gyroscope 1 further comprises a first pair of acceleration sensors with axis parallel to the vertical axis z, and in particular a first pair of first sensing masses 16a, 16b, set within a respective first through opening 9a, 9b so as to be completely enclosed and contained within the overall dimensions of the driving mass 3 in the plane of the sensor xy. Each of the first sensing masses 16a, 16b has a shape corresponding to that of the respective through opening, and consequently has, in a plan view, the general shape of a radial annulus sector. In detail, each of the first sensing masses 16a, 16b has a first portion 17, which is wider, and a second portion 18, which is narrower (along the first horizontal axis x), these portions being connected by a connecting portion 19, which is shorter (in a direction parallel to the second horizontal axis y) than the first and second portions 17, 18, and consequently has a center of gravity G located within the corresponding first portion 17. In greater detail, the first portion 17 has an outer side that is arc-shaped and concave, and radially-extending lateral sides, and the second portion 18 has an outer side that is arc-shaped and convex and radially-extending lateral sides, aligned along the lateral sides of the first portion 17. Each of the first sensing masses 16a, 16b is supported by a pair of first elastic supporting elements 20, extending from the connecting portion 19 to the driving mass 3, connecting thereto, parallel to the second horizontal axis y. The first elastic supporting elements 20 extend within recesses 21 provided on opposite sides of the corresponding first sensing mass 16a, 16b, at a distance from its center of gravity G. The first elastic supporting elements 20 form torsional springs which are rigid in regard to the rotary motion of the driving mass 3 (so that the first sensing masses 16a, 16b follow the driving mass 3 in its motion of actuation), and moreover enable rotation of the first sensing masses about an axis of rotation parallel to the second horizontal axis y and belonging to the plane of the sensor xy, and hence their movement out of the plane of the sensor xy (a movement that is not, instead, allowed for the driving mass 3).
The gyroscope 1 further comprises a second pair of acceleration sensors with axis parallel to the vertical axis z, and in particular a second pair of first sensing masses 16c, 16d, housed within the through openings 9c, 9d, and completely enclosed and contained by the driving mass 3. The first sensing masses 16c, 16d are obtained from the rotation through 90° of the first sensing masses 16a, 16b with respect to the center O, and consequently the corresponding elastic supporting elements 20 extend parallel to the first horizontal axis x and enable rotation out of the plane of the sensor xy, about an axis of rotation parallel to the first horizontal axis x.
A pair of first and second sensing electrodes 22, 23 is set underneath the first and second portions 17, 18 of each of the first sensing masses 16a-16d. The first and second sensing electrodes 22, 23 are made of polycrystalline-silicon regions formed on the substrate 2a and having a substantially trapezoidal shape and dimensions substantially corresponding to those of the second portion 18. The first and second sensing electrodes 22, 23 are separated, respectively, from the first and second portions 17, 18, by an air gap, and hence form, together with the first and second portions 17, 18, respective sensing capacitors. The first and second sensing electrodes 22, 23 are connected to a read circuit of the gyroscope 1 (not illustrated) via the connection pads 2d. 
The gyroscope 1 further comprises a pair of second sensing masses 25a, 25b housed within the second through openings 26a, 26b. The second sensing masses 25a, 25b have a generally rectangular shape with sides parallel to corresponding sides of the second through openings 26a, 26b, are suspended with respect to the substrate 2a, and are connected to the driving mass 3 via second elastic supporting elements 28. The second elastic supporting elements 28 depart, for example, from a point set approximately at the center of the minor sides of the second sensing masses, in the radial direction. In particular, the second elastic supporting elements 28 are rigid with respect to the motion of actuation of the driving mass 3 (in such a way that the second sensing masses 25a, 25b follow the driving mass 3 in its rotary movement), and moreover enable a linear movement of the respective second sensing masses in the aforesaid radial direction. In addition, the second sensing masses 25a, 25b have prolongations 29 extending, for example, starting from a point set approximately at the center of corresponding major sides, in a direction orthogonal to the radial direction. These prolongations 29 form sensing capacitors with plane and parallel faces with fixed electrodes anchored to the substrate, set facing, and parallel to, the same prolongations 29. For example, from each major side of each second sensing mass 25a, 25b departs a respective prolongation 29, which faces, and is set between, two fixed electrodes. In a way similar to what has been described previously, the fixed electrodes set in a radially more external position with respect to the center O are defined as “first sensing electrodes 22”, and the fixed electrodes set in a radially more internal position are defined as “second sensing electrodes 23”.
In use, the gyroscope 1 is able to operate as a triaxial gyroscope, and to detect a pitch angular velocity {right arrow over (Ω)}x about the first horizontal axis x, a roll angular velocity {right arrow over (Ω)}y about the second horizontal axis y, and a yaw angular velocity {right arrow over (Ω)}z about the vertical axis z.
With reference also to FIG. 2, the rotary movement of the driving mass 3 and of the first sensing masses 16a-16d about the axis of actuation can be represented by a driving-velocity vector {right arrow over (v)}a, tangential to the circumference describing the path thereof. In particular, the rotary motion about the first horizontal axis x or the second horizontal axis y with angular velocity {right arrow over (Ω)}x, {right arrow over (Ω)}y causes a Coriolis force (designated by {right arrow over (FC)}) acting on the entire structure, proportional to the vector product between the angular velocity {right arrow over (Ω)}x, {right arrow over (Ω)}y; and the driving velocity {right arrow over (v)}a, and hence directed along the vertical axis z. On the entire structure, considered as a single rigid body, it is hence possible to identify a distribution of Coriolis forces, the value of which increases as the distance from the center O increases. The resultants of the Coriolis force {right arrow over (F)}C acting on the first sensing masses 16a-16d at the corresponding center of gravity G, cause rotation of the first sensing masses, which move out of the plane of the sensor xy, about an axis parallel to the first horizontal axis x or the second horizontal axis y and passing through the first elastic supporting elements 20. This movement is allowed by the torsion of the first elastic supporting elements 20. Instead, the configuration of the first and further elastic anchorage elements 8a, 8b is such as to inhibit, to a good approximation, the movement of the driving mass 3 out of the plane of the sensor xy, in this way enabling the effective decoupling of the motion of detection of the sensing masses with respect to that of actuation. The displacement of the first sensing masses 16a-16d out of the plane of the sensor xy causes a differential capacitive variation of the sensing capacitors, the value of which is proportional to the angular velocity {right arrow over (Ω)}x, {right arrow over (Ω)}y, which can thus be determined, via an appropriate read circuit, operating according to a differential scheme.
In particular, since the reading scheme is differential, the configuration in pairs of the first sensing masses 16a-16d enables automatic rejection of linear spurious accelerations along the vertical axis z. In addition, a rotation about the first horizontal axis x is not felt by the second pair of first sensing masses 16c, 16d, in so far as the resultant Coriolis force {right arrow over (F)}C is zero (since the vector product between the angular velocity {right arrow over (Ω)}x and the corresponding driving velocity {right arrow over (v)}a goes to zero). Likewise, the rotation about the second horizontal axis y is not felt for similar reasons by the first pair of first sensing masses 16a, 16b so that the two axes of detection are not affected and are substantially decoupled.
With reference to FIG. 3, an angular velocity {right arrow over (Ω)}z to be detected, acting about the vertical axis z, generates a Coriolis force {right arrow over (F)}C on the second sensing masses 25a, 25b set in a radial direction (hence directed as a centrifugal force acting on the same masses), causing displacement of the second sensing masses and a capacitive variation of the corresponding sensing capacitors. The value of the capacitive variation is proportional to the angular velocity {right arrow over (Ω)}z, which can be determined, via the read circuit, operating again according to a differential scheme.
Advantageously, the particular conformation of the first sensing masses 16a-16d enables an increase in the sensitivity of the gyroscope 1 (as compared to the use of other geometries for the same first sensing masses). In particular, the corresponding center of gravity G is positioned at a distance from the first elastic supporting elements 20 (and the corresponding axis of rotation out of the plane of the sensor xy) that is greater than that of the center of gravity of any rectangular mass that can be inscribed in one and the same sector of the driving mass 3 and is supported by elastic supporting elements extending along the same axis of rotation. Consequently, it is possible to obtain a higher twisting moment, and hence a greater movement of rotation out of the plane of the sensor xy, and in this way obtains a higher sensitivity of the sensor.
In addition, the presence of the further elastic anchorage elements 8b, located outside the driving mass 3, enables increase of the stiffness of the driving mass 3 in regard to the movements out of the plane of the sensor xy, and so an increase in the decoupling between the driving movement and the movements of detection.
Even though the described gyroscope represents a considerable improvement over other gyroscopes of a known type, it is not altogether optimized in respect of manufacturing simplicity, size reduction, efficiency in terms of electrical characteristics, and immunity to disturbance.